Method of producing oxygen-free high conductivity copper



United States Patent M t 3,298,070 METHOD OF PRODUCING OXYGEN-FREE HIGH CONDUCTIVITY COPPER William J. Yurko, Matawan, N .J., and Dennis K. Pickens, Darien, Conn assignors to Chemetals Corporation, New York, N.Y., a corporation of Delaware Filed Aug. 13, 1965, Ser. No. 479,537 7 Claims. (Cl. 22214) This application is a continuation-in-part of the pending application Serial No. 300,603, filed August 7, 1963, now abandoned.

This invention relates to theproduction ofoxygenfree copper and more particularly, relates to a process of producing oxygen-free copper from copper powders.

Oxygen-free copper is a well-known and valuable articleof commerce. The term oxygen-free does not necessarily mean that the copper is completely free of oxygen but it means a copper product which has substantially all of the oxygen removedtherefrom and thus possessing a very high conductivity. Oxygen-free copper, as defined by A.S.T.M. standards is one in which the resistivity after annealing is less than about 0.15238 international ohm-gram/meter when measured at. 20

C. Oxygen-free high conductivity copper is presently being produced by a number of companies employing electrolytic cathodes as starting materials. The process involves the melting of the cathodes under a reducing atmosphere after substantially all of the contained oxygen and other impurities have been removed, and sub sequently casting the melted copper under a reducing oi inert atmosphere into the desired form preferably wire bars. t

. Copper powders are generally dendritic in nature and the exposed surface area of the particles is very large and quite active. The removal of oxygen from the fine copper powder particles while still in the solid state and having these largesurface areas exposed results in a product in which the level of oxygen content as well as other impurities such as carbon and sulfur content, if present, is considerably lower than anything thus far achieved on a commercial scale. As a result of the considerably lower levels of oxygen obtainable by this process, as well as other impurities such as carbon and sulfur,

the copper produced has unique characteristics including excellent electrical conductivity and formability. The ductility of the copper is unchanged by cold deformation and it can thus undergo successive drawings without intermediate annealing. The oxygen-free nature of the copper also makesit immune to embrittlement by exposure to a hydrogen bearing atmosphere at elevated temperatures. In addition to the high quality oxygenfree wrought copper produced by the process of this invention, the process itself can be operated continuously and is ideally suited for commercial production and is highly efficient and economical.

The process of this invention involving the removal of the oxygen from .copper powder in substantially particulate form will not only remove substantially all of the oxygen contained therein, but will also remove other volatile impurities particularly carbon and sulfur which may be present in the copper powder particles as sulfates, chlorides, nitrates, and so forth. These volatile impurities are reduced by the reducing gas -such as hydrogen leaving an oxide present in the copper which is subsequently reduced by contact with the reducing gas. 1

One important aspect of the invention concerns the fact when the copper in particulate form is heated in a bers or slugs 3.

3,298,070 Patented Jan. 17, 1967 reducing atmosphere such as hydrogen, the reducing gas is adsorbed by the individual copper particles. The copper particles when melted evolve the contained hydrogen or rerducing gas. The combination of heating the individual particles in a reducing gas atmosphere and the melting of the particles with evolution of the reducing gas such as hydrogen from each individual particle gives an extremely uniform and complete reduction in removal of volatile impurities and oxygen from the copper being treated. Thus, whatever oxygen or other volatile impurities that might be left in the powder is substantially completely uniformly dispersed throughout the casting substantially eliminating embrittlement problems and producing a cast copper product having other advantageous and improved properties such as tensile strength, elongation and conductivity. This is, of course, in contrast to the conventional method of bubbling a reducing gas through a copper melt which has the disadvantage of producing wire bars which are not uniform throughout their cross section with respect to purity content but have localized areas containing high percentages of oxygen and other impurities.

Another important advantage of the process of this invention is that copper of lower quality (containing a higher percentage of oxygen and volatile impurities) can be used as a starting material to produce a high grade oxygen-free high conductivity copper which would not be possible by the processes heretofore employed.

Other advantages of the invention will be pointed out in the following description and claims and illustrated in the accompanying drawings, which disclose, by way of example, the principles of the invention and the preferred embodiments thereof for applying those principles. In the drawings:

FIGURE 1 is a diagrammatic representation of the apparatus of one embodiment of the invention which employsa reducing gas; and

FIGURE 2 is a diagrammatic representation of the apparatus of another embodiment of the invention which employs a reducing gas and an inert gas.

Referring to FIGURE 1, the copper powder containing oxygen is evenly distributed on a traveling belt 1 of commercial muflie type of heated furnace generally indicated in the drawing by the numeral 2. The copper powder is retained in sections by the belt dividing mem- The" belt continuously conveys the copper powder through the indirectly heated muffle of the furnace. -The heating elements of the furnace are designed to heat the copper powder to a temperature of between about 1000 F. and 1900 F. The copper powder is thus gradually heated as it enters the furnace and reaches the temperature desired, 1750 F. for example before it reaches the exit end of the furnace designated generally at 4 in the drawing. The copper powder particles at this high temperature fuse together to form a fairly self-supporting form, low density slab, but one in which the particles retain their identity as such. The reducing atmosphere is supplied by a forward flow of reducing gas through the furnace as shown by the arrows 5. Thefreducing gases contact the copper powder particles throughout their travel through the furnace. The reducing gas used is preferably hydrogen and when hydrogen is used the oxidized copper is reduced by the hydrogen to produce water which is removed from the furnace by the forward flow of the hydrogen through the furnace. The reducing gas or hydrogen is allowed to burn in the combustionhood at the entrance to the furnace thereby 3 providing partial heat or a pre-heat for drying and for heating the incoming copper.

The copper powder can be supplied to the belt 1 in particulate form or in the form of briquettes. If briquettes are used, care should be taken to see that they are lightly compacted and porous to allow for maximum oxygen removal when passed through the muffie furnace containing the reducing atmosphere. If briquettes are used as a feed to the mufile furnace, they are preferably cold compacted and should not be sintered. In view of the fact that copper powder in particulate form presents a greater surface area than copper powder in briquette form, it is advantageous to deposit the copper powder on the belt 1 in the particulate or powder form.

It is also an advantage of this process that the copper powder can be deposited on to the belt 1 for transfer through the mufile furnace containing the reducing atmosphere in the wet state. This is particularly important when the process of the invention is utilized for the production of oxygen-free copper from hydrometallurgically produced copper powders which are recovered from the hydrometallurgical process in the wet state. No intermittent drying step of the wet hydrometallurgical produced powder is required and as pointed out above, the heat generated by the reducing gases at the combustion hood at the entrance of the furnace can provide sufiicient heat for the drying of the copper powder before it enters the muffle furnace. This thus, permits a continuous process in which copper powder can be produced by a hydrometallurgica-l process such as by reduction in an autoclave with hydrogen and the wet copper powder continuously and directly transeferred to the belt 1.

The temperature of the muffle surface can be varied quite widely but is advantageously designed so that the copper will be heated between 1000 F. and 1900 F. prior to its exit from the muffle furnace. The specific temperature employed will, of course, depend upon the particular copper powder being processed, the length of the muffle furnace, time of heating, and so forth. Sufficient heat should be used, however, to insure the substantially complete removal of the oxygen and volatile impurities from the copper powder and to partially fuse the copper powder into a porous low density self-supporting slab in which particle indentity is maintained. Excessive heat should also be avoided to prevent the melting of the copper particles or the sintering thereof to a substantially dense or wrought form. If the slab exiting the furnace becomes too dense and approaches wrought form, all of the oxygen might not be removed and the hydrogen gas adsorbed by the copper particles would substantially be removed prior to the transformation of the particles from the solid to the liquid state. The copper slab exiting the muffle furnace should thus be of such a nature that the majority of the copper particles will substantially retain their individual identity as particles.

As the copper slabs 6 leave the muffle furnace, they enter directly into a seal box. The copper slabs drop from the seal box directly into the electric induction furnace heating by induction heating to a temperature of approximately 2150 F. plus or minus 20 F. Fluctuations of this magnitude are caused by heat losses and entry of the copper slabs into the furnace. The copper slabs enter the induction furnace for melting prior to any significant reduction of the temperature of the slabs from the time they leave the muffle furnace until the time they enter the induction melting furnace. In this manner, the adsorbed hydrogen in the copper powder particles is retained and is only released during the transformation of the particles from the solid to the liquid state.

Temperature variations of plus or minus 20 F. constitute a problem in continuous casting and therefore, there is need for a pouring furnace in which the temperature of the molten metal can be controlled more accurately. The molten metal contained in the induction melting furnace, as shown in the drawing, is thus transferred to the pouring furnace through an electrically heated closed launder. The launder is located at the axis of rotation of the pouring furnace and the two are connected by a seal which permits the furnace to rotate but prevents leakage of gas. The metal is heated by rod elements, not shown, as it flows through the launder and the rate of discharge of the molten metal to the closed launder is governed by the voltage applied. The pouring furnace can be tilted forward and at the same time can be moved horizontally along rails. The pouring furnace is also heated by electric induction and both the pouring and melting furnaces are automatically controlled by thermocouples and electric pyrometers.

The reducing gas or hydrogen can be supplied to the entire system by cracking ammonia, reforming hydrocarbons or electrolysis of water, or by other suitable means, and is supplied, as shown in the drawing to both furnaces and the heated launder. Both furnaces and the heated launder are kept under slight pressure by the reducing gas so that the reducing gas flows from the pouring furnace through the launder into the melting furnace, through the seal box and into the heated muffle and finally through the combustion hood and up the stack.

The pouring of the molten copper from the pouring furnace into the casting mold can be regulated manually by a plug (not shown). The molten copper can be continuously cast under a protective atmosphere of reducing gas into wire bars from 2" x 2" to 6 x 6" in crosssections and in lengths adjustable up to at least about 60". The main components of the casting machine, as shown in the drawing, are the casting table with the chill mold and water spray, the transport rolls and the saw preferably provided with a hydraulic mechanism for cutting the wire bar. After the wire bar has been cut by the saw, it is deposited into a basket which in turn can deposit the Wire bar onto a wire bar conveyor for subsequent use.

The process and the equipment described including the electric induction melting furnace, the launder, the pouring furnace, the components of the casting machine, are all conventional and well-known and, therefore, have not been set forth in the drawing or described herein in detail.

The temperature used in the mufile furnace will also depend upon the type and amounts of other volatile impurities in the copper powder such as carbon and sulfur. If the copper powder contains an undesirable amount of other impurities such as carbon and sulfur, the temperature of the muffle furnace should be such that the copper powder or the copper slabs increase in temperature on the traveling belt through the furnace to a point where such impurities as carbon and sulfur are removed from the copper powder or slab as organic compounds and hydrogen sulfide.

In another embodiment of the invention, which is shown in FIGURE 2, the copper particles or compacts are cooled to a reduced temperature of about 300 F. prior to being melted and cast into wire bars. As the copper slab-s leave the heated muffle furnace, they enter into a cooling chamber, as shown in FIGURE 2, and are cooled in an inert atmosphere composed primarily of about to nitrogen (N prior to being melted in the electric induction melting furnace. As described hereinabove, in the other embodiment, the oxygen and other volatile materials are substantially removed from the copper in the heated muflle furnace in a reducing atmosphere of hydrogen. The copper in this embodiment, however, is melted and cast in an inert atmosphere, preferably nitrogen.

Even though the cooling step results in the loss of heat, the cooling step does have a number of advantages. When the copper is melted in a reducing atmosphere of hydrogen, the hydrogen is capable of being adsorbed into the liquid copper and when the copper is cast, the adsorbed hydrogen is released from the copper during the casting operation. The release of even a small amount of the hydrogen from the copper may cause blisters in the copper. However, in this embodiment of the invention, when the copper slabs are cooled in an inert atmosphere consisting essentially of nitrogen, a vacuum is formed within the porous copper slabs which draws the nitrogen into the copper. The nitrogen replaces the hydrogen in the void areas of the porous copper slabs and prevents the contamination of the copper in the molten stage.

The cooling chamber and seal box may be sealed off from the heated mufile furnace by any conventional well known means to prevent any undue contamination of the inert atmosphere with the hydrogen.

Thetemperature within the cooling chamber may be reduced by. any suitable known means so long as the copper particles are cooled to a reduced temperature of at least about 300 F.

According to this embodiment of the invention, the inert gas, nitrogen, is supplied to the cooling chamber, seal box, launder, melting furnace and pouring furnace through the cooling chamber, whereas, the reducing gas, hydrogen, is supplied to the muflle furnace. The different gases are supplied from similar but separate sources, as shown in the drawing, in a manner as described hereinabove, in the other embodiment of this invention.

In the hot embodiment of this invention, an inert gas could also be used in the seal box, launder, melting furmace and pouring furnace provided the oxygen is removed from the oxygen by the hydrogen in the heated muffle furnace.

Various types of copper powder can be used to produce the oxygen-free high conductivity copper according to this invention, but the process is particularly advantageous when using hydrometallurgically-produced powders as starting materials. The metallic impurities of copper powders will not significantly effect the removal of oxygen therefrom but the presence of metallic impurities may effect the physical characteristics and electrical conductivity of the resultant wrought copper. Thus, the starting copper powder used as a starting material should be fairly pure and if volatile impurities such as sulfur and carbon are present, they should be present in amounts sufficiently low so that they can be substantially removed within a reasonable time when processed according to this invention. The technology of producing copper powders by hydrometallurgy is well known and the copper powders produced according to such a process are generally sufficiently pure so that there will be no interference in the production of the oxygen-free high conductivity copper according to this invention. Other methods of producing copper powders which can be used according to this invention besides pressure hydrometallurgically include shotting, abrasion, chemical precipitation and so forth. Copper oxide powders can also be used.

Examplel A copper powder was produced by hydrogen reduction from an aqueous ammoniacal solution having the following screen analysis:

Mesh: Percent The copper powder had an apparent density of 2.57 grams per cc. and a flow rate of 28.8 sec./ 50 gm. The Fisher Sub-Sieve Size was 13.4 microns. The flow rate was determined by the Hall method. This method is known and merely involves the use of a box having a small hole of predetermined diameter in the middle. Into this box is placed 50 grams of powder and the time it takes to flow out of the box determines the flow rate. The copper powder was analyzed and found to have the following approximate composition on a percent by weight basis: t

Percent Copper 99.812 Tin r 0.0001 Iron -1..- 0.006

Sulfur 0.019

Carbon 0.015

Zinc 0.014

Insoluble 0.01 Lead 0.001

Molybdenum 0.001 Hydrogen loss 0.11

The copper powder was formed] into briquettes by cold compression using only enough pressure to form a selfsubstaining briquette while substantially retaining the particle identity. i

The copper briquettes were then heated in a confined areasuch as that represented by the separator elements as shown in the drawing at a temperature of 1750 F. for 1 hour while being maintained under a reducing atmosphere of hydrogen gas. The heating ofthe copper powder particles in briquette form as described above resulted in the removal therefrom of substantially all of the oxygen contained therein as well as a significant amount of the volatile impurities. The briquette after being heated at 1750 F. for 1 hourwas also found to contain a substantial amount of hydrogen gas absorbed into the copper particles forming the briquette.

The heated briquette while it was. still retained at substantially the same temperature used to remove theoxygen and other volatile impurities and containing the adsorbed hydrogen was melted by induction heat at a temperature of about 2190" F. in a suitable furnace. After the melted copper particles had reached a constant temperature of 2190 F. the molten copper was poured into a casting mold and consolidated therein to form a wire bar. The melting of the briquettes also resulted in an evolution of the adsorbed hydrogen from the copper powder particles forming the briquette.

The cast bars were rolled into rod and the rod processed through draw benches down to 0.014 wire. The resulting wire had a conductivity percent volume at 20 C. of 100.52%.

Another portion of the melted powder was cast into wire bars and rolled into 7 inch rods and drawn to .081 diameter wire.

The rods were tested and found to have a tensile strength of 32,000 psi. and an elongation percent of 40 (10 in.). A wire (annealed at 1000 F. for l hour) was tested and found to have a tensile strength of 34,900 p.s.i., a percent elongation of 38 (10 in.) and a conduc tivity percent volume at 20 C. of 100.022.

The rods produced as described above were also cold worked to near full hardness and then annealed at 527 F. for 15 minutes. The maximum desired hardness of the 30 T scale is 20. The hard rods before annealing had a value of 65.5 to 63.0 30 T scale. After annealing the rods were found to have a value of 15.5 to 16.5 30 T scale.

The resulting copper rods and wire performed well in tensile, elongation, conductivity and annealed ability.

Example 11 The procedure of Example I was followed except that prior to being melted in the melting furnace, the copper briquettes were passed into a cooling chamber, such as shown in the drawing, and were cooled to a reduced temperature of about 300 F. in an inert atmosphere composed of about to nitrogen (N (which was the atmosphere of the entire system with the exception of the muffle furnace, combustion hood and stack which had a reducing atmosphere of hydrogen).

The results were similar to those obtained in Example I and there were no signs of any blisters on the resulting copper rods.

We claim:

1. The process of producing oxygen-free wrought copper which comprises heating copper powder in which the copper powder particles contain oxygen and volatile impurities at a temperature between about 1000 F. and 1900 F. in a reducing atmosphere for a sufiicient length of time to remove substantially all of the oxygen and volatile impurities contained in the copper powder particles and also to cause adsorption of the reducing gas into the copper powder particles, melting the copper powder particles prior to any significant reduction of temperature in 'a protective atmosphere to prevent the contamination thereof with oxygen and while the copper powder particles still contain a substantial amount of adsorbed reducing gas to cause release of the adsorbed reducing gas from the copper powder particles while they are changing from the solid to the liquid state and subsequently casting the oxygen-free copper in a protective atmosphere to maintain the copper substantially free of oxygen.

2. The process of claim 1 in which the reducing gas contains hydrogen.

3. The process of claim 2 in which the copper powder particles are consolidated into a substantially self-supporting form prior to the melting thereof.

4. The process of claim 1 in which the copper powder is produced from an aqueous ammoniacal solution by reduction with hydrogen.

5. The process of claim 1 in which the copper powder is a copper oxide powder.

6. The process of producing oxygen-free wrought copper which comprises heating copper powder in which the copper powder particles contain oxygen and volatile impurities at a temperature between about 1000 F. and 1900 F. in a reducing atmosphere for a sufiicient length of time to remove substantially all of the oxygen and volatile impurities contained in the copper particles and also to cause adsorption of the reducing gas into the copper powder particles, cooling the copper powder particles to a reduced temperature of about 300 F. in a protective atmosphere of an inert gas to prevent the contamination thereof with oxygen and where a substantial amount of the adsorbed reducing gas is released which is replaced with the inert gas within the copper particles, melting the copper powder particles at a sufliciently high temperature in a protective atmosphere of a partially reducing gas to maintain the copper substantially free of oxygen and subsequently casting the oxygen-free copper in a protective atmosphere of a partially reducing gas to maintain the copper substantially free of oxygen.

7. The process of claim 6 in which: (a) the inert gas comprises about to nitrogen and (b) the reducing gas contains hydrogen.

References Cited by the Examiner UNITED STATES PATENTS 1,057,882 4/1913 Rockey et a1 7576 2,452,996 11/ 1948 Corson 22-214 2,805,149 9/ 1957 Schaufelberger 7591 FOREIGN PATENTS 317,400 12/ 1930 Great Britain.

1. SPENCER OVERHOLSER, Primary Examiner.

MARCUS U. LYONS, E. MAR, Assistant Examiners. 

1. THE PROCESS OF PRODUCING OXYGEN-FREE WROUGHT COPPER WHICH COMPRISES HEATING COPPER POWDER IN WHICH THE COPPER POWDER PARTICLES CONTAIN OXYGEN AND VOLATILE IMPURITIES AT A TEMPERATURE ABOUT 1000*F. AND 1900*F. IN A REDUCING ATMOSPHERE FOR A SUFFICIENT LENGTH OF TIME TO REMOVE SUBSTANTIALLY ALL OF THE OXYGEN AND VOLATILE IMPURITIES CONTAINED IN THE COPPER POWDER PARTICLES AND ALSO TO CAUSE ADSORPTION OF THE REDUCING GAS INTO THE COPPER POWDER PARTICLES, MELTING THE COPPER POWDER PARTICLES PRIOR TO ANY SIGNIFICANT REDUCTION OF TEMPERATURE IN A PROTECTIVE ATMOSPHERE TO PREVENT THE CONTAMINATION THEREOF WITH OXYGEN AND WHILE THE COPPER POWDER PARTICLES STILL CONTAIN A SUBSTANTIAL AMOUNT OF ADSORBED REDUCING GAS TO CAUSE RELEASE OF THE ADSORBED REDUCING GAS FROM THE COPPER POWDER PARTICLES WHILE THEY ARE CHANGING FROM THE SOLID TO THE LIQUID STATE AND SUBSEQUENTLY CASTING THE OXYGEN-FREE COPPER IN A PROTECTIVE ATMOSPHERE TO MAINTAIN THE COPPER SUBSTANTIALLY FREE OF OXYGEN. 